mendelian genetics


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mendelian genetics

  1. 1. Mendelian Genetics Chapter 2
  2. 2. Phenotype and Genotype
  3. 3. Genotype and Phenotype• Genotype – genetic constitution of an organism• Phenotype – observable characteristic • Genotype and environment • Contribution of environment varies between genes • Can be controlled by many genes • Random developmental events
  4. 4. Mendel’s Experimental Design
  5. 5. Mendelian Genetics• Modern genetics began with Gregor Mendel’s quantitative genetic experiments• Austrian monk• Mathematician• Numerical and observational data• Several generations Stamen Carpel
  6. 6. Mendelian Genetics• Heritable, obvious traits• Simple crosses at first• Used peas because: • Easy to grow and available • Many distinguishable characteristics • Self-fertilization• True breeding peas
  7. 7. Pea Traits
  8. 8. Monohybrid Crosses and Mendel’s Principle of Segregation
  9. 9. Breeding Crosses• Initial cross is the P generation • Parents• Progeny of parents is first filial generation • F1 generation• Inbreeding of first generation creates second filial generation • F2 generation
  10. 10. Monohybrid Crosses• Cross between true- breeding individuals with one different trait• Mendel’s first crosses• Resembled only one of the parents• Planted progeny and allowed self-fertilization • Revealed both phenotypes
  11. 11. Monohybrid Cross• Mendel determined that P PLANTS GENETIC MAKEUP (ALLELES) PP pp • Particulate factors for Gametes All All genes, each contains a set P p of two F1 PLANTS All Pp (hybrids) • Transmitted by both parents Gametes 1 /2 P 1 /2 p • Alternate forms called alleles Eggs P P P Sperm F2 PLANTS p P p • True breeding forms Phenotypic ratio 3 purple : 1 P p P p contains identical set white Genotypic ratio 1 PP : 2 Pp : 1 pp p p
  12. 12. Monohybrid Cross• F1 generation had both alleles• Only one expresses• One allele masks • Dominant • Recessive• Identical alleles – homozygous• Different alleles - heterozygous
  13. 13. Monohybrid Cross
  14. 14. Principle of Segregation• Recessive characteristics are masked • Reappear in F2• Members of a gene pair (alleles) segregated during gamete formation
  15. 15. How cells carry characteristics• Genes on chromosomes • At a specific loci• Homologous pairs carry the same genes at the same locus • Different versions• Separation of homologous chromosomes yields separation of alleles
  16. 16. Branch Diagrams• Punnett squares can become messy with more than one gene• Use branch diagram to figure out genotype and phenotype expected frequency
  17. 17. Test Cross• Mendel did several TESTCROSS: crosses • Followed over several GENOTYPES B_ bb generations Two possibilities for the black dog:• Selfing also very BB or Bb important • Allowed plants to GAMETES B B b reveal their genotype and not just their b Bb b Bb bb phenotye OFFSPRING All black 1 black : 1 chocolate
  18. 18. TestCross
  19. 19. Recessive Alleles• Wild-type allele – functional allele • Predominates in population • Dominant allele• Loss-of-function mutations – causes protein product to be absent, partially functional, or nonfunctional • Recessive • Function of other in heterozygote is sufficient
  20. 20. Wrinkled Peas• SS type contains more starch and lower sucrose • Also more water • SBEI - starch- branching enzyme • Extra 800 bp piece in mutation
  21. 21. Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent Assortment
  22. 22. The Principle of IndependentAssortment• Factors for different traits assort independently of one another • Genes are inherited independently of each other • Segregate randomly in gametes• Dihybrid Cross
  23. 23. Branch Diagram of Dihybrid CrossPhenotype
  24. 24. Genotype vs.Phenotype
  25. 25. Test Cross With Dihybrid
  26. 26. Trihybrid Cross
  27. 27. Tribble Traits Activity
  28. 28. Statistical Analysis of Genetic Data: The Chi-Square Test
  29. 29. Statistical Analysis• Data from genetics is quantitative• Use statistics to show deviation of observed results from predicted results • Chance factors cause deviations• Null-hypothesis – no difference between the predicted and observed • If not accepted then have to come up with a new hypothesis for deviation
  30. 30. Chi-Square Test• Goodness of fit test • How much observed number deviates from the expected number
  31. 31. Mendelian Genetics in Humans
  32. 32. Pedigree Analysis• Inheritance patterns are studied using family trees • Pedigree analysis • Phenotypic records • Proband is where gene was discovered
  33. 33. Examples of Human Genetic Traits• Most genetic disorders are recessive • Due to lack of function• Homozygous recessive expression • Dominant usually selected out• Albinism
  34. 34. Characteristics of RecessiveInheritance Traits• Most have normal heterozygous parents• Heterozygotes have 3:1 ratio• When both parents have the trait then all progeny have the trait• Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs
  35. 35. Characteristics of DominantInheritance Traits• Gain of function mutations • New property of the mutant gene • No loss of function• Must have one parent with disease• Does not skip generations• Will transmit to half its progeny • Huntingtons disease, Marfan syndrome, achondroplasia